Active dihedral control system for a torisionally flexible wing

ABSTRACT

A span-loaded, highly flexible flying wing, having horizontal control surfaces mounted aft of the wing on extended beams to form local pitch-control devices. Each of five spanwise wing segments of the wing has one or more motors and photovoltaic arrays, and produces its own lift independent of the other wing segments, to minimize inter-segment loads. Wing dihedral is controlled by separately controlling the local pitch-control devices consisting of a control surface on a boom, such that inboard and outboard wing segment pitch changes relative to each other, and thus relative inboard and outboard lift is varied.

The present invention relates to aircraft. More particularly, thepresent invention relates to aircraft having unique control mechanisms,and related methods of controlling an aircraft. The present applicationis a divisional application of U.S. patent application Ser. No.11/732,109, filed Apr. 2, 2007, now U.S. Pat. No. 7,802,756, which is acontinuation in part of U.S. patent application Ser. No. 10/310,415,filed Dec. 5, 2002, now U.S. Pat. No. 7,198,225, which is a divisionalapplication of U.S. patent application Ser. No. 09/527,544, filed Mar.16, 2000, now abandoned, which claims priority from U.S. provisionalpatent application Ser. No. 60/182,165, filed Feb. 14, 2000, each ofwhich is incorporated herein by reference for all purposes. U.S. patentapplication Ser. No. 11/732,109, filed Apr. 2, 2007, now U.S. Pat. No.7,802,756, is also a continuation in part of U.S. patent applicationSer. No. 10/600,258, filed Jun. 20, 2003, now U.S. Pat. No. 7,281,681,which is a continuation in part of U.S. patent application Ser. No.10/073,828, filed Feb. 11, 2002, now abandoned, which is a divisional ofU.S. patent application Ser. No. 09/826,424, filed Apr. 3, 2001, nowU.S. Pat. No. 6,550,717, which claims priority from U.S. provisionalapplication Ser. No. 60/241,713, filed Oct. 18, 2000, and which alsoclaims priority from U.S. provisional application Ser. No. 60/194,137,filed Apr. 3, 2000, each of which is incorporated herein by referencefor all purposes.

This invention was made with government support under ERAST JSRAContract NCC-04004 awarded by NASA. The United States Government hascertain rights in the invention.

BACKGROUND

Aircraft are used in a wide variety of applications, including travel,transportation, fire fighting, surveillance and combat. Various aircrafthave been designed to fill the wide array of functional roles defined bythese applications. Included among these aircraft are balloons,dirigibles, traditional fixed wing aircraft, flying wings andhelicopters.

One functional role that a few aircraft have been designed to fill isthat of a high altitude platform. Operating from high, suborbitalaltitudes, such aircraft can monitor weather patterns, conductatmospheric research and surveil a wide variety of subjects.

Three high altitude aircraft that have been constructed are thewell-known Pathfinder, Centurion and Helios aircraft, which have setnumerous flight records. The basic design concepts underlying theseaircraft are discussed at length in U.S. Pat. No. 5,810,284, which isdirected toward an unswept flying wing aircraft having a very highaspect ratio and a relatively constant chord and airfoil. While theseaircraft are quite noteworthy for their long term flight potential, theydo have limits in their available power and payload.

Such aircraft may be designed as flying wings that include a number ofself-sufficient wing sections, each generating enough lift to supportits own weight. To minimize weight, the aircraft structure is highlyflexible, and is designed to withstand only relatively small torsionalloads and moderate bending loads along its lateral axis (i.e., itswingspan). The aircraft's wing has little or no dihedral while on theground. However, due to high flexibility, the large aspect ratio and theconstant chord, in-flight wing loads tend to cause the wing to develop asubstantial dihedral angle at the wingtips, which may not be optimal fora given wing strength. Thus, there is a tradeoff between the structuralweight of the aircraft and the desirability of the wing shape.

There is an inherent relationship between an aircraft's overall airframegeometry and the design of its airfoils and control surfaces. Typicalaircraft offset negative (i.e., nose-down) pitching moments through theuse of tail moments (i.e., vertical forces generated on empennagehorizontal surfaces and elevators, with a moment arm that is thedistance from the wing center of pressure to the empennage verticalcenter of pressure).

To minimize the torsional loads, the Pathfinder, Centurion and Heliosaircraft include “wing-mounted elevators” along a substantial portion oftheir trailing edges (i.e., the trailing edges of each flying wingsegment). These aircraft do not include rudders or ailerons, and thewing-mounted elevators are not designed as elevons (i.e., they cannotmove in contrary directions near opposite wingtips). Roll is passivelycontrolled by the dihedral of the wing, which is developed in flight.Sideslip is also passively controlled by the dihedral of the wing. Asdiscussed above, the allowable wing dihedral is limited by thestructural strength of the wing.

Given the broad range of functions that a long-duration, suborbitalplatform has the potential to perform, it is desirable to design suchhigh-altitude platforms to be capable of handling larger payloads andpower demands. The platforms could be variations of existing platforms,such as larger variations of the Pathfinder, Centurion and Heliosaircraft, but such platforms will likely have to handle increasedbending loads along the wing as such larger aircraft have to reactagainst dihedral-causing forces over a larger wingspan.

There exists a definite need for a multipurpose aircraft that can remainairborne for long durations. Preferably, such an aircraft should be ableto operate up to very high, suborbital altitudes. Importantly, it isdesirable for such an aircraft to have the capability to meet largerpayload and/or power supply requirements. Furthermore, there exists aneed for such an aircraft to be structurally light weight and wellcontrolled. Various embodiments of the present invention can meet someor all of these needs, and provide further, related advantages.

SUMMARY OF THE INVENTION

The present invention addresses the needs mentioned above by providingan aircraft that can operate at high altitudes, carry substantialpayloads, and/or remain aloft for long periods of time.

The aircraft of the invention typically includes a laterally extendingwing, a plurality of pitch-control devices, and a control systemconfigured to control the plurality of pitch-control devices. Eachpitch-control device is mounted at a separate lateral location along thewing. Each pitch-control device is configured to apply pitch-controltorque at its lateral location, and the wing is characterized by atorsional flexibility high enough for each pitch-control device toseparately and substantially control localized pitch at its lateral winglocation, i.e., to a degree substantial enough to be significant forflight control.

The pitch-control device may feature a body, e.g., a boom, connectingthe wing to a control surface aft of the trailing edge of the wing.Advantageously, the control surface is positioned at a distance from thewing adequate to provide the aerodynamic forces from the control surfacewith a pitching effect on the wing to cause changes in the local liftthat dominate (i.e., are much larger than) the changes in lift thatoccur from the redirection of air by the control surface (i.e., the flapeffect), over the entire flight envelope. Thus, aileron reversal is notan issue.

The invention further features that the control system is configured tooperate the pitch-control devices under protocols that will activelycontrol wing dihedral. Advantageously, under such predeterminedprotocols, a highly flexible wing can be used while limiting the risk ofexcessive wing bending.

Other features and advantages of the invention will become apparent fromthe following detailed description of the preferred embodiments, takenin conjunction with the accompanying drawings, which illustrate, by wayof example, the principles of the invention. The detailed description ofparticular preferred embodiments, as set out below to enable one tobuild and use an embodiment of the invention, are not intended to limitthe enumerated claims, but rather, they are intended to serve asparticular examples of the claimed invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view of an aircraft embodying the invention.

FIG. 2 is a plan view of the aircraft depicted in FIG. 1.

FIG. 3 is a perspective view of the aircraft depicted in FIG. 1, in aflexed position that creates moderate dihedral typical of loading undermild flight conditions.

FIG. 4 is a perspective, cutaway view showing the construction of oneportion of one wing segment of the wing of the aircraft depicted in ofFIG. 1.

FIG. 5 is a block diagram showing a control system and relatedcomponents from the aircraft illustrated in FIG. 1.

FIG. 6 is a partial plan view of a second aircraft embodying theinvention.

FIG. 7 is a partial plan view of a third aircraft embodying theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention summarized above and defined by the enumerated claims maybe better understood by referring to the following detailed description,which should be read in conjunction with the accompanying drawings. Thisdetailed description of a particular preferred embodiment, set out belowto enable one to build and use one particular implementation of theinvention, is not intended to limit the enumerated claims, but rather itis intended to serve as a particular example thereof.

In accordance with the present invention, a number of preferredembodiments of an aircraft of the present invention are of designssimilar to those of the Pathfinder, Centurion and/or Helios aircraft, asmentioned above in the Background of the Invention. While theembodiments' designs, and variations of them, are described below,further details useful for the practicing of this embodiment of theinvention are provided in U.S. Pat. No. 5,810,284, which is incorporatedherein by reference for all purposes. Nevertheless, it is to beunderstood that designs for other embodiments of the invention candiffer substantially from the described aircraft.

Like the Pathfinder, Centurion and Helios aircraft, the preferredembodiments may be flying wings. These embodiments include a pluralityof laterally connected, wing segments that preferably can each supporttheir own weight in flight so as to minimize inter-segment loads, andthereby minimize required load-bearing structure. These embodiments haveaircraft control systems configured to control the flexible developmentof wing dihedral during flight, and thereby further controlinter-segment loads.

The Pathfinder, Centurion and Helios aircraft had trailing-edge controlsurfaces configured as trailing edge flaps (or “wing-mounted elevators”on the trailing edge of the wing). These control surfaces were notconfigured to act differentially. The coordination of the wing trailingedge control surfaces to prevent contrary movement on different portionsof the wing was not utilized. The torsional flexibility of thePathfinder, Centurion and Helios aircraft made the use of such controlsurfaces relatively impractical. Lacking the torsional rigidity of anormal aircraft, the Pathfinder, Centurion and Helios aircraft couldsuffer from significant control reversal problems if the controlsurfaces operated as ailerons. Under some circumstances, thesedifficulties also might affect the operation of the control surfaces aselevators. Thus, the control reversal issue potentially limited theoperability of the aircraft.

For example, a downward control surface deflection on a normal,torsionally stiff wing, would typically be expected to cause additionalairfoil section lift (an effect that will be hereinafter referred to asa “flap effect”). However, such a deflection will likely cause asignificant nose-down pitching (twisting) moment on the wing, which on atorsionally flexible wing can lead to a decreased angle of attack, andthereby a reduction in overall lift (an effect that will be hereinafterreferred to as a “pitch effect”). Under various flight conditions, acontrol surface on the trailing edge of a torsionally flexible wing canexperience one, the other and/or both of these two contrary effects to asignificant degree.

As a result, the response to a movement of the control surface on ahighly flexible (in torsion) winged aircraft can be unpredictable.Moreover, over the flight envelope (e.g., through variations in flightspeed), the response can vary between having one of the effectsdominate, having the other dominate, having the two cancel each otherout, and having the two cyclically operate with one lagging the other todrive the wing in a potentially unstable forced vibration (i.e.,flutter) having both bending and torsional components.

With reference to FIGS. 1-3, a first preferred embodiment is a flyingwing aircraft 10, i.e., it has no fuselage or empennage usable tocontrol the overall pitch of the aircraft (as a typical aircraft wouldhave). Instead, it consists of an unswept, laterally extending wing 12similar to that of the Centurion aircraft, having a substantiallyconsistent airfoil shape and size along the wingspan. Fourteen motors 14are situated at various locations along the wingspan, each motor drivinga single propeller 16 to create thrust. Four vertical fins 18 a-18 d, orpods, extend down from the wing, with landing gear at their lower ends.

The aircraft 10 is longitudinally divided into preferably five modularwing segments sequentially located along the lateral wingspan. Theseinclude a center segment 20, left and right intermediate segments 22,24, and left and right wingtip segments 26, 28. These wing segmentspreferably range from 39 to 43 feet in length, and have a chord lengthof approximately eight feet. Alternative variations of the embodimentmay be highly flexible flying wing aircraft that are unitary (i.e., notsegmented), but are nevertheless highly flexible.

With reference to FIGS. 2, 3 and 4, one or more of the wing segments ofthe aircraft 10, and preferably at least three wing segments (asdepicted) (and/or up to and including all of the wing segments) eachinclude a pitch-control device 42, each pitch-control device beingmounted at a separate lateral location along the wing. The pitch-controldevice is preferably a boom 44 extending longitudinally aft and holdinga preferably horizontal control surface 46 in a position preferably aftof the trailing edge of the wing 12. For the purposes of thisapplication, it should be understood that a “horizontal” surface is oneextending in a direction having a horizontal component, that isadequately horizontal to impart control forces having a relevantvertical component. In alternative embodiments, the pitch-control devicecould include both a fixed horizontal surface and an active controlsurface.

The three wing segments having pitch-control devices are preferably aninboard wing segment (e.g., the center segment 20) and two outboard wingsegments (e.g., the end segments 26, 28). Thus, the flying wingpreferably includes at least 3 pitch control devices, which arepreferably located symmetrically across the wing.

Each such pitch-device control surface 46 is configured for rotationallydeflecting relative to the boom 44 such that a controllable, preferablyvertical aerodynamic force is applied to the boom aft of the trailingedge of the wing. The force applied to the boom is preferably normal tothe longitudinal dimension of the boom, and at a distance from the wingsegment on which it is mounted, such that a torsional force is appliedto the wing segment at or about the lateral location to which the boomstructurally connects to the wing segment.

Moreover, the wing 12 is characterized by enough torsional flexibilityin the lateral locations of each pitch-control device 42 to separatelycontrol localized pitch of the wing at and/or near its lateral winglocation. In this application, the terminology “separately control”should be understood to mean that the pitch-control devices arephysically independent such that each could in theory be commanded tooperate in a manner different from the others.

This control over localized pitch is to a degree substantial enough tobe significant for flight control (i.e., for control of the response ofthe aircraft structure to aerodynamic forces, so as to change theaircraft structural configuration (e.g., wing dihedral and/or bendingload) and/or the aircraft flight or orientation). The position andconfiguration of each pitch-control device preferably limits any flapeffect it has on the wing segment (in response to deflection of thecontrol surface) such that the pitch effect is dominant over the entireflight envelope of the aircraft. In other words, the change in verticalforce from movements of the pitch-control device control surface, aresignificantly less than the change in lift experienced by the wing dueto the resulting change in local wing pitch.

Each pitch-control device boom 44 connects the control surface 46 to thewing 12 at a distance aft of both the spar 40 and the trailing edge 48of the wing adequate to cause the control surface pitch effect todominate the control surface flap effect. This is distinctive from anormal aircraft, for which wing-mounted control surfaces are intended tooperate using a dominant flap effect.

Optionally (as depicted in FIG. 4), additional, flap-effect controlsurfaces 50 could be incorporated into the trailing edge of the wing,particularly in locations structurally close to (e.g., within a spanwisearea torsionally affected and/or controlled by) a pitch-control device42. These trailing-edge control surfaces could be limited in use toflight regimes where in their response would be predictable, or could beused in concert with a pitch-control device to produce desired effects(e.g., the trailing edge control surface could control lift while thepitch control device limits the wing pitch resulting from movements ofthe trailing edge control surface). Alternatively, the pitch-controldevices may be the only control surfaces (or the only horizontal controlsurfaces) on the aircraft.

The overall length of the pitch-control device as measured back from theelastic axis of the wing, and its control surface size, may beexperimentally or analytically determined to meet the criteria ofminimizing overall weight and drag, while providing for the pitch effectto be the dominant effect over the entire desired flight envelope.Possible pitch-device lengths that might be considered, as multiples ofthe wing fore-and-aft length (i.e., chord length), include 1.5 and 3.

Thus, the aircraft of this embodiment might have a chordwise length ofroughly 20 feet, with a wing segment chordwise length of eight feet, anda wingspan of approximately 200 feet. The structure is configured to belightweight, with significant flexibility in vertical bending (allowingfor significant dihedral bending) and spanwise torsion (allowing forsignificant relative pitching).

With reference to FIGS. 2, 4 and 5, the embodiment includes anelectronic aircraft control system 52 configured to control theoperation of the aircraft. The aircraft control system includes astructural control system 54 configured to control structural bending ofthe aircraft, and a flight control system 56 configured to control theflight of the aircraft. Because these two functions may be significantlyinterrelated, the structural control system and flight control systemare likely to significantly interact within the overall aircraft controlsystem 52.

Both the structural control system 54 and the flight control system 56receive data from numerous sources. One such source is a communicationsunit 61 configured to receive instructions from a ground controller(e.g., a ground-based pilot). Another source is a plurality of flightparameter sensors 63, preferably including one or more of the followingsensors: a positional sensor (e.g., a GPS), a heading sensor, a pitchsensor, a roll sensor, a yaw sensor, an altimeter, a flight speedsensor, a vertical speed sensor, a slip sensor, a pitch rate sensor, aroll rate sensor, and a yaw rate sensor. A third source is a pluralityof structural sensors 65, preferably including one or more of thefollowing sensors: vertical wing bending sensors, fore-and-aft wingbending sensors, wing torsion sensors, motor speed and/or thrustsensors, control surface deflection and/or force sensors, and solarsensors configured to detect the exposure of the structure to sunlight.Each of these sensors is of a type either known in the art (e.g., straingauges and positional sensors), or that can be formed with a combinationof known sensors.

In some cases, one or more sensors of one type may serve the function ofthe sensor of another type. For example, a plurality of pitch sensorsand/or pitch rate sensors laterally positioned along the wing mayprovide data to analytically determine wing torsion, which mightotherwise be detected with strain gauges.

The structural control system 54 and the flight control system 56 mayeach contribute to command instructions sent to a number of aircraftsystems. The systems receiving command instructions to control theiroperation include the control surfaces (e.g., pitch-control devicecontrol surfaces 46, and flap-effect control surfaces 50) and themotors. As noted above, in some cases the structural sensors will be ofa type to sense the operation of the control devices (e.g., the controlsurfaces and/or the motors).

Using the aircraft control system 52 and the pitch-control devices 42,aircraft dihedral is controlled by having the structural control system54 cause aircraft control system commands to be sent to thepitch-control devices to initiate control movements of their controlsurfaces 46 using a protocol that controls the pitch of their respectivelateral locations on the wing, and relatedly affect their wing segmentsand/or nearby portions thereof (and possibly the pitch of nearby wingsegments). In particular, outboard pitch-device control surfaces 72 aredirected to actuate downward (i.e., trailing edge down), causing theirrespective wing segments 26, 28, or portions of their respective wingsegments to pitch downward (i.e., leading edge down) and therebydecrease the overall lift generated by the respective outboard wingsegments.

Simultaneously, inboard pitch-device control surfaces 74 are directed toactuate upward, causing their respective wing segments, or portions oftheir respective wing segments 20 to pitch upward and thereby increasethe overall lift generated by the respective inboard wing segments. As aresult, with inboard lift increased and outboard lift decreased, overallwing dihedral may be controllably reduced, eliminated, and/or controlledto achieve desired wing configurations and desired wing stress levels.

The aircraft control system is thereby configured to control theplurality of pitch-control devices under a protocol (i.e., a detailedplan or procedure) that controls wing dihedral according to apredetermined program. Such a program will typically include dihedrallimits (e.g., maximums dictated by flight efficiency and structurallimits, and optionally minimums dictated by flight control issues,possibly varying over the entire flight envelope), and dihedralschedules (such as ones based on maximizing the exposure of wing solarcells to sunlight, ones based on optimizing the positions of onboardinstrumentation, or ones based on stability and control parameters). Theprotocol may include control inputs that are symmetric, such as ones toincrease or decrease dihedral, control inputs that are inverted onopposite sides, such as ones to roll the aircraft, and possibly evencontrol inputs that are asymmetric.

In order to optimize flight efficiency by reducing drag, the aircraftcontrol system dihedral schedule may be configured (i.e., the protocolmay include command procedures) to cause the dihedral to be less whenthe sun is high in the sky, or when it is night. This allows theaircraft to optimize the tradeoff between power generation and flightefficiency. To accomplish this end, the control system determines adihedral configuration to increase the power generated by solar cells,should they be present. This can be done by simply reading a clocksignal from a clock within the aircraft control system and adjusting thedihedral (and possibly the heading) based on the anticipated lightconditions. More preferably, the control system can detect the lightconditions, either through signals from light sensors, or fromindications of the power levels generated by one or more of the solarcells.

As suggested above, in some situations it might be desirable to increasewing dihedral. To do so, the reverse of the above-recited operation isconducted. More particularly, outboard pitch-device control surfaces 72are directed to actuate upward, causing their respective wing segments,or portions of their respective wing segments, to pitch upward andthereby increase the overall lift generated by the respective outboardwing segments. Simultaneously, inboard pitch-device control surfaces 74are directed to actuate downward, causing their respective wingsegments, or portions of their respective wing segments, to pitchdownward and thereby decrease the overall lift generated by therespective inboard wing segments.

As a result of the above design, the preferred embodiment of theaircraft is light, travels at relatively slow air speeds, and has aconfiguration controllable to limit stresses on its individualcomponents. Optionally, the control system may receive input fromsensors configured to detect the configuration (e.g., the relativeposition, orientation, bending and/or torsion) of the aircraft and/orindividual wing segments thereof. Thus, the aircraft control system mayactively control the aircraft configuration to be maintained withinstructural safety limits (e.g., for the bending stresses to bemaintained within safety limits) and within an optimum flightconfiguration range, even when the aircraft encounters undesirableflight conditions such as turbulence.

Preferably the pitch-control devices 42 are each paired with (i.e.,located substantially aft of) a motor 14, thus potentially limiting theeffects of drag from the pitch-control device on the wing 12 (i.e., theuse of paired motors and pitch-devices limits the shear forces andfore-and-aft bending of the wing due to moment arms between the thrustof the nearest motor(s) and the drag of the pitching device). Thedepicted outboard pitch-control devices are paired with motors.Optionally, the wing may include additional motors that are not pairedwith pitch-control devices (as depicted for the inboard pitch-controldevice). The motors may optionally be controlled by a motor controlsystem 58, (which may be part of the aircraft control system) that isconfigured to control the operation of the motors such that the unpairedmotors (i.e., motors not paired with a pitch-control device) areoperated at a lower thrust level than the paired motors, the differencebeing at or about the anticipated or actual level of pitch-device drag,which may vary by flight condition and control surface position.Likewise, two or more motors near an unpaired pitch-control device maybe controlled by the aircraft system controller to provide relativelyincreased thrust in a proportional amount based on their lateralpositions relative to the pitch-control device.

As a result, the motor control system is configured to separably controlthe thrust from the plurality of motors to reduce fore-and-aft wingloads between the motors. Optionally, the motor control system mayoptimize this function using flight data and sensory informationregarding wing strain, actual thrust and actual structural configuration(e.g., wing bending, wing torsion and other related parameters).

The aircraft 10 controls yaw, and thereby turns, using differentialthrust from varied motor torque on the propellers 16. It uses acombination of sideslip and dihedral to control bank angle. Optionally,the pitch-control devices could be used to create varied lift over thewingspan, and thereby control bank angle without large side slip issues.Other known methods or mechanisms for creating differential thrust couldalso be used.

The aircraft relies upon its large wingspan and relatively lowvelocities to avoid yaw instability. Roll may be controlled passively bythe wing being maintained with a positive angle of dihedral, and/or byusing the pitch-control devices to create differential lift across thewingspan.

The aircraft may further include inter-segment hinge mechanisms andhinge locks, as described in U.S. patent application Ser. No.10/310,415, filed Dec. 5, 2002, which is incorporated herein byreference for all purposes. The structural control system may furthercontrol the pitch-control devices to actuate the inter-segment hingemechanisms (i.e., acting as hinge actuators), as described in thatapplication. The hinge locks (i.e., hinge-rotation locks) can be eitherwithin the hinge mechanisms, or otherwise controlling them. When arotational lock is in an unlocked configuration, hinge actuators allowthe relative rotation of respective wing segments. When the rotationallock is in a locked configuration, the hinge mechanism is restrained,and the respective wing segments are prevented from rotating withrespect to each other, thereby maintaining the wing's dihedralconfiguration.

The aircraft may optionally feature additional, non-aerodynamicmechanisms (as described in the above-noted application), configured toaffect the local wing pitch (i.e., pitch-control devices) and/or tocontrol the rotation of the hinge mechanisms, thereby adding furthercontrollability to the wing configuration and/or the operation of thehinge mechanisms. These mechanisms may include CG-movement devices(i.e., devices configured to change the center of gravity in aparticular area of the wing so as to affect its pitch and/or roll). Itis preferable that there be a symmetric arrangement of hinge mechanismson the aircraft, along with a symmetric arrangement of pitch-controldevices.

Additional configurations, such as aircraft configured to deflect intoW-shapes or M-shapes are also within the scope of the invention. Suchconfigurations having alternating positive and negative dihedral canreduce wing loading for flight conditions in which it is desirable tohave significant side exposure of the wing surfaces (such as when thesun is low on the horizon). Furthermore, aircraft with only twopitch-control devices or only one pitch-control device are also withinthe possible scope of the invention, particularly when combined with astructural control system implementing protocols as described above.

While the described embodiments of active dihedral control are employedon an aircraft having numerous, flexible, non-swept wing segments ofconstant airfoil and chord, they can likewise be employed on otheraircraft designs including conventional aircraft, and even biplanes.

More particularly, with reference to FIG. 6, another embodiment may be aconventional aircraft provided with a flexible wing 401, which supportsa fuselage 403, and includes a number of highly flexible regions 405capable of significant independent wing torsion. Each region has apitch-control device 407 that controls the pitch of that region, andreacts any negative pitching moments of that region's cambered airfoil.The aircraft wing 401 will preferably include at least one pitch-controldevice 407 on each side of the fuselage 403 in a symmetric formation.Preferably (though not necessarily), the fuselage carries an empennage(not shown) that includes typical horizontal control surfaces, and/orother fuselage-mounted pitch-control surfaces (e.g., a canard).

Preferably, the primary function of the pitch-control devices 407 iscontrolling and/or preventing local wing torsion and bending, butoverall flight control can also be a primary or secondary function.Overall aircraft pitching moments can also be reacted by thefuselage-mounted pitch-control surfaces. An aircraft control systempreferably controls both the pitch-control devices and anyfuselage-mounted pitch-control surfaces to those ends, and preferablyreceives input from various sensors, as described with reference to thefirst embodiment.

While the above-described pitch-control devices actively control localwing pitch, another embodiment of the invention uses passive controls(i.e., pitch-limiting devices) so as to allow the use of ailerons on ahighly flexible wing without experiencing aileron reversal. While anaircraft with a fuselage is described in the embodiment below, otherembodiments may be of other configurations, such as flying wings likethose described above.

With reference to FIG. 7, another embodiment may be a conventionalaircraft provided with a highly flexible laterally extending wing 501,which supports a fuselage 503, and includes a number of highly flexibleregions 505 capable of significant independent wing torsion. A pluralityof ailerons 506 are mounted at various lateral aileron-locations in thehighly flexible regions along the wing.

A plurality of pitch-limiting devices 507 are mounted at separatelateral pitch-limiting-locations along the wing. Each pitch-limitingdevice is configured to apply a pitch-limiting torque at itspitch-limiting-location. Each pitch-limiting-location is proximate theaileron-locations of one or more ailerons. Thus, each region has apitch-limiting device 507 that limits the pitch of that region inresponse to aileron deflection. The aircraft wing 501 will preferablyinclude at least one pitch-limiting device 507 on each side of thefuselage 403 in a symmetric formation. Preferably (though notnecessarily), the fuselage carries an empennage (not shown) thatincludes typical horizontal control surfaces, and/or otherfuselage-mounted pitch-control surfaces (e.g., a canard).

It should be understood that a wing that is uniformly (and highly)flexible can be considered as having a number of highly flexibleregions. The term highly flexible should be understood to represent alevel of torsional flexibility wherein but for any pitch-limitingdevices (i.e., if they weren't there), one or more ailerons wouldexperience aileron reversal over some portion of the flight envelope.

While the pitch-limiting devices could be active horizontal controlsurfaces controlled by a control system to limit wing pitch, or acombination of a control surface and a fixed horizontal surface,preferably the pitch-limiting devices include only one or more fixedhorizontal surfaces mounted aft of the wing. More particularly, eachpitch-limiting device preferably includes a body (e.g., a boom)connecting the wing to a fixed surface aft of the trailing edge of thewing at a distance adequate to cause the flap effect of the proximateailerons to dominate the pitch effect over the entire flight envelope.The primary function of the pitch-limiting devices 507 is controllingand/or preventing local wing torsion and bending, and thereby allowingailerons to function properly without experiencing aileron reversal.

Advantageously, the features described above with respect to the variousembodiments can provide various advantages. By allowing for hightorsional flexibility, torsion-carrying wing structure can be limited,reducing the weight of the aircraft and thereby potentially increasingits payload capacity. Moreover, by controlling wing bending loads, wingspar weight can be reduced. Furthermore, by providing control over thestructure, potentially expanded flight envelopes are available to theaircraft. Improved stability and control may be obtainable usingcontrolled wing shape (e.g., dihedral), as well as improved fluttercharacteristics (which again provide for expanded flight envelopes).Moreover, the increased structural weight of the devices may bepartially offset by the elimination of ailerons and/or wing-mountedelevators.

From the foregoing description, it will be appreciated that the presentinvention provides a number of embodiments of a lightweight aircraftcapable of both stationkeeping and flight over a wide range of speeds,while consuming low levels of power, for an extended period of time,while supporting an unobstructed communications platform, and whileexhibiting simplicity and reliability

Other embodiments within the scope of the invention include devicescomprising forward extending booms configured with canards, andCG-movement devices. Likewise, other embodiments of the invention couldhave other numbers of wing segments, including variations with an evennumber of wing segments (e.g., six wing segments), and other numbers ofmotors. For example, an embodiment similar to the Helios aircraft mightbe configured with six wing segments, 10 motors, and anywhere from twoto six (or possibly more) independent pitch-control devices. Likewise, asimple embodiment might include three wing segments with one to threemotors and two or three (or perhaps even one) independent pitch-controldevices, or might even be a very long unsegmented wing with one or moremotors and a plurality of independent pitch-control devices.

While a particular form of the invention has been illustrated anddescribed, it will be apparent that various modifications can be madewithout departing from the spirit and scope of the invention. Thus,although the invention has been described in detail with reference onlyto the preferred embodiments, those having ordinary skill in the artwill appreciate that various modifications can be made without departingfrom the invention. Accordingly, the invention is not intended to belimited by the above discussion, and is defined with reference to thefollowing claims.

We claim:
 1. A flying wing aircraft, comprising: a laterally extendingwing having a spanwise axis; wherein the wing has high torsionalflexibility about the spanwise axis; a plurality of independentlyactuatable control surfaces; wherein the control surfaces are spacedapart along the span of the wing and connected thereto; a plurality ofsensors connected to the wing; and a control system programmed toactively control the plurality of control surfaces based upon structuralinformation data received from the plurality of sensors; wherein thecontrol system independently actuates the control surfaces to alterlocal lift of wing portions at the location of each control surface inorder to alter the dihedral of the wing.
 2. The flying wing aircraft ofclaim 1, wherein the wing comprises a plurality of interconnected wingsegments.
 3. The flying wing aircraft of claim 1, wherein each controlsurface is a pitch-control device mounted on a boom extending aft from atrailing edge of the wing.
 4. The flying wing aircraft of claim 1,wherein the plurality of sensors include at least one of a vertical wingbending sensor, a fore-and-aft wing bending sensor, and/or a wingtorsion sensor.
 5. The flying wing aircraft of claim 1, furthercomprising: a plurality of motors connected to the wing; and a pluralityof propellers connected to the motors; wherein the control systemindependently controls the thrust of the plurality of motors in order toalter fore-and-aft bending of the wing in order to alter fore-and-aftwing loads between the motors.
 6. The flying wing aircraft of claim 5,wherein each control surface is a pitch-control device mounted on a boomextending aft from a trailing edge of the wing; and wherein one of theplurality of motors and propellers is connected to a leading edge of thewing at the spanwise location of each of said booms.
 7. The flying wingaircraft of claim 1, wherein the control surfaces comprise fixed andcontrollable horizontal surfaces.
 8. The flying wing aircraft of claim1, further comprising: a plurality of solar cells connected to the wing;wherein the solar cells provide power to the aircraft for flight.
 9. Anaircraft, comprising: a fuselage; a laterally extending wing having aspanwise axis; wherein the wing has high torsional flexibility about thespanwise axis; a plurality of independently actuatable control surfaces;wherein the control surfaces are spaced apart along the span of the wingand connected thereto; a plurality of sensors connected to the wing; anda control system programmed to actively control the plurality of controlsurfaces based upon structural information data received from theplurality of sensors; wherein the control system independently actuatesthe control surfaces to alter local lift of wing portions at thelocation of each control surface in order to alter the dihedral of thewing.
 10. The aircraft of claim 9, wherein the wing comprises aplurality of interconnected wing segments.
 11. The aircraft of claim 9,wherein each control surface is a pitch-control device mounted on a boomextending aft from a trailing edge of the wing.
 12. The aircraft ofclaim 9, wherein the plurality of sensors include at least one of avertical wing bending sensor, a fore-and-aft wing bending sensor, and/ora wing torsion sensor.
 13. The aircraft of claim 9, further comprising:a plurality of motors connected to the wing; and a plurality ofpropellers connected to the motors; wherein the control systemindependently controls the thrust of the plurality of motors in order toalter fore-and-aft bending of the wing in order to alter fore-and-aftwing loads between the motors.
 14. The aircraft of claim 13, whereineach control surface is a pitch-control device mounted on a boomextending aft from a trailing edge of the wing; and wherein one of theplurality of motors and propellers is connected to a leading edge of thewing at the spanwise location of each of said booms.
 15. The aircraft ofclaim 9, wherein the control surfaces comprise fixed and controllablehorizontal surfaces.
 16. The aircraft of claim 9, further comprising: aplurality of solar cells connected to the wing; wherein the solar cellsprovide power to the aircraft for flight.
 17. A method of controlling anaircraft comprising the steps of: providing an aircraft having: alaterally extending wing having a spanwise axis; wherein the wing hashigh torsional flexibility about the spanwise axis; a plurality ofindependently actuatable control surfaces; wherein the control surfacesare spaced apart along the span of the wing and connected thereto; aplurality of sensors connected to the wing; a control system programmedto actively control the plurality of control surfaces based uponstructural information data received from the plurality of sensors; andindependently actuating the control surfaces to alter local lift of wingportions at the location of each control surface to alter the dihedralof the wing.
 18. The method of controlling an aircraft according toclaim 17, wherein the wing has a large aspect ratio and a constant chordlength; wherein the high torsional flexibility, large aspect ratio, andconstant chord length cause the wing to develop a substantial dihedralangle at the wingtips in flight, further comprising the steps of:directing outboard control surfaces to actuate downwardly; directinginboard control surfaces to actuate upwardly; and deflecting the wing inthe spanwise direction to reduce and/or eliminate wing dihedral.
 19. Themethod of controlling an aircraft according to claim 18, furthercomprising the step of: directing outboard control surfaces to actuatedownwardly; directing inboard control surfaces to actuate upwardly; anddeflecting the wing in the spanwise direction to produce a negativedihedral.
 20. The method of controlling an aircraft according to claim17, further comprising the steps of: independently actuating theplurality of independently actuatable control surfaces to alter thelocal lift of wing portions to deflect the wing into a W-shape orM-shape along the spanwise axis.